1. Technical Field
The present invention relates to a light source device, a method of driving a discharge lamp, and a projector.
2. Related Art
As a light source of a projector, a discharge lamp such as a high pressure mercury lamp and a metal halide lamp is used.
In the discharge lamp, an alternating current as a driving current is supplied between a pair of electrodes to cause arc discharge such that a discharge medium enclosed in the discharge lamp emits light. When the discharge lamp is turned on, the electrodes are at a high temperature by the arc discharge generated between the pair of electrodes, the electrodes are melted, and a space between the electrodes is broadened.
In the use of a projector, in order to improve efficiency in light usage, it is preferable to keep a narrow state between the electrodes to lower the intensity of light emission. It is not preferable that the electrodes are separated from each other during ignition, whereby the efficiency of light usage is decreased. The change between the electrodes means a change in impedance between the electrodes. For this reason, even when the discharge lamp is efficiently turned on at the initial time of ignition, impedance mismatching occurs after the time has elapsed. As a result, there is a problem that reactive power is increased, and the efficiency is decreased.
For example, in a driving method disclosed in JP-A-2004-172086, a voltage between a pair of electrodes of a discharge lamp is detected, a frequency of an alternating current is lowered when the inter-electrode voltage is lower than the lower limit value, protrusions formed at front end portions of a pair of electrodes are melted, an inter-electrode distance is increased, and thus an inter-electrode voltage is lowered.
When the inter-electrode voltage is higher than an upper limit value, the frequency of the alternating current is raised, the protrusions formed at the front end portion of a pair of electrodes are grown, the inter-electrode distance is decreased, and the inter-electrode voltage is lowered.
However, in the driving method disclosed in JP-A-2004-172086, the frequency of the alternating current is 400 Hz even in the maximum, blackening or devitrification of the discharge lamp body occurs, and durability of the discharge lamp may be decreased.
In order to grow the protrusions formed at the front end portions of a pair of electrodes, it is preferable to set the frequency of the alternating current to be equal to or lower than 1 kHz. However, when such a low frequency alternating current is supplied to a pair of electrodes, the discharge lamp body is more easily blackened.